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The actual single 1.8GHz is a whole different motherboard than the dual, and it's not possible to add a second one.

The older single 1.6GHz and 1.8GHz have the same board layout than the dual. When you open it you can see the space for the second CPU simply being free.
Unfortunately, Apple did not put the 2nd CPU Connector on those boards, and surely other electronics are missing for it too...so even on those models it is not possible to add a second CPU.

Unlike the dual G4, the dual G5 does not share 1 bus with 2CPUs, but simply each CPU has it's own bus and memory access. therefore I really doubt that there will be any such upgrade. And AFAIK, the actual single 1.8GHz has it's CPU soldered on the board, not sitting on a daughterboard like the others...

So you'll never be able to upgrade the processor on a G5, wow !! :bomb:

That's actually a different question. And actually an interesting one.

My original question was can I add another processor and the answer seems
to be no short of a change in motherboards and maybe some other stuff.

Upgrading the processor is different question altogether.

One other question would be: can I overclock this thing? With $100 processors,
that's an interesting question as frying the thing isn't expensive. With $500
processors, the question becomes a little heated.

I was responding to what Avalon said about the CPU being soldered to the board and not the daughterboard, I mean if this is true then it's going to make a processor upgrade very expensive.

For the soldered one (which, for the moment, means iMac G5 and the recent PowerMac G5 single 1.8) upgrading the CPU is not an option...simply buying a new one would be more reasonable and cheaper.

But the older singles (1.6 and 1.8) are not soldered, and could take a faster CPU. The only problem is that G5s don't get their speed in the same way than other CPUs (including G3, G4, AMD and Intel).
The other systems all have an FSB that's somewhat slow compared to the CPU speed, and the CPU gets it's clockspeed by multiplying the FSB. This means you can put in a CPU that get's multiplied higher, and thus runs faster. Overclocking a CPU is nothing else than setting that multiplier to higher settings.

Now the G5 is a whole different story...it's FSB is much higher (depending on the model, half or a third of CPU speed), and it is different between the different PowerMacs. The dual 1.8GHz has a 900MHz FSB, while the dual 2.5GHz has a 1.25GHz FSB. That means even if you would put the 2.5GHz CPUs in a dual 1.8, they wouldn't run faster at all. The G5s with soldered CPU have a slower FSB (single 1.8GHz has 600MHz FSB), but even here it's the same rule. The difference between the iMac 1.6 and the 1.8 is the higher FSB, so interchanging CPUs wouldn't change anything (and would require some extremely good soldering skills too).

I don't know in which way the G5 multiplier works, but as the FSB is much higher, overclocking isn't really an option. For example, if you take the dual 2GHz, it has a 1GHz FSB, so it's a 2x multiplier. Now setting this multiplier to 3x would, theoretically, boost the CPUs to 3GHz. The big problem here is that overclocking is nothing else than using the CPU manufacturer's tolerances, which are limited, of course. No CPU can take such a high speed increase, except with some highly sofisticated cooling solutions (water cooling would NOT be enough).

Another option would be to overclock the FSB, but this is not recommended...it would overclock your whole system, and chances are extremely low that it will run stable. When you overclock the CPU you only play with one component's tolerances, which works as long as you don't exceed the CPUs limits. But on the whole system are many different parts, that each have their own tolerances and limits. This also includes extension cards like PCI or video card, which would be overclocked too. That's why increasing the FSB in a significant way would surely not work, and probably make your system useless.

Didn't expect the Imacs to be upgradable but then the nice thing about the iMacs is that you can afford to replace them every once in a while, I expected a lot more upgradability with the Powermacs as this is one of the reason why I'd buy one.